The present invention is with respect to a diaphragm for a press for producing plates and other workpieces, having a rigid upper press ram, whose form is designed answering to the inner form of the workpiece to be pressed, and with a pressure medium cavity shut off at the top by the diaphragm, which, after the lowering of the press ram onto powder china material, with a powder distribution on the diaphragm within its limits as fixed by its supported edge, in the pressing position of the pressing ram, the cavity being in a form at whose edge the edge, having a hooked cross-section, of the diaphragm is fixedly kept in position by way of an inner part, running into the hooked cross-section, and a keeper ring which may be placed on this system and fixed against the mold edge.
A plate making press using such a diaphragm is to be seen for example in the German Auslegeschrift specification No. 2,627,160.
Plate workpieces as made with such a press have a pointed edge which is produced by pressing conditions and which has to be taken off using fettling tools so that the plate edge has the desired rounded form.
Such later processing was generally not possible in the case of plates with festooned edges, that is to say with an edge curving inwards and outwards round the plate.
The reason for the pointed or wedge-like edge is that the pressed plates, after the pressing operation, undergo an increase in size when the pressure is let off, with the outcome that the plate's outer diameter is increased so that, if the plate edge is gripped by the diaphragm or other press parts, it is much harder for the plate to be lifted upwards and it may be damaged on doing so. If, on the other hand, the plate edge is designed to be wedge-like, the forces which have to be overcome before the plates may be taken from the diaphragm are very much smaller, but however such wedge-like plate edges are undesired in the completed product and the edges have to be rounded off by fettling, much material being taken from the plate on doing this. In the case of a festooned edge, cutting material from the plate is generally not possible, more specifically in the inwardly curving parts so that plate presses which have so far been put forward are in need of further development in this respect.